Laboratory measurement guide to External Optical Modulation with Mach-Zehnder Interferometer to the subjects of Building Blocks of Optical Networks (Neptun code: BMEVIHVMA05) and Lightwave Systems and Applications (Neptun code: BMEVIHVMA06)

Abstract

A brief explanation of the theory is given in this measurement guide. For more details, please see the References [1-7]. Optical Intensity Modulation (IM) means that the output optical power of the light source is modulated in order to be able to transmit analog or digital information signals optically. IM is very similar to electronic Amplitude Modulation (AM) in Radio Frequency (RF) domain. However, there is a basic difference: amplitude (either voltage, current or electric field) can be negative. But output optical power and intensity can never be negative. Optical power is -by definition- positive. Intensity modulation is shown in the figure below. The output power of the Laser Diode (LD) can be modulated directly by changing the bias current if the LASER diode. The main difference between direct and external modulation is shown below. Direct modulation changes directly the bias current of the light source. Such the output power is altered. In case of external modulation, the LD emits an unmodulated continuous wave (CW) signal, and an external component, an optical intensity modulator is inserted into the optical path. The laser diode is the optical source, responsible for providing a λ wavelength. The analog or digital modulation is added by the modulator, that changes the intensity of the laser diode.